How to Test Trailer Lights

No matter what type of trailer you tow, trailer lights are an important component of safe and legal towing. In case when something goes wrong and your trailer lights don’t work properly or don’t work at all, you should fix them immediately. Before you would call on all of your knowledge about how to test trailer lights and begin to do it, you may want to check your trailer’s light as the answer may be as simple as a burned-out bulb.

If you haven’t found it, you have to ensure that your vehicle’s brake and backup lights as well as turn signals work properly and the problem doesn’t occur in the tow vehicle. For this purpose you will have to disconnect the wiring system of your trailer from your tow vehicle as you won’t know for certain whether the problem is in the wiring harness. Testing your tow vehicle without the trailer will allow for easier testing by separating the wiring system into two manageable sections.

If your vehicle’s lights operate as they should, you will need a trailer wiring tester that allows for easy checking of the socket on your tow vehicle. Normally, each wiring tester has one or more indicators which will alert you when detecting an electric current. Use this tool to test each function (running, right, left, and stop lights.) If you have the light illuminating for each system, then the wiring system of your trailer is likely to be the source of the problem. But when any of tests results in no response from the wiring tester, your tow vehicle’s wiring should be checked. You may also find out that the tester lights up when it shouldn’t.

For instance, if you haven’t engaged the turn signal, but any of your signal indicators lights up, you may have a short in your vehicle’s wiring. So you will have to check your tow vehicle’s wiring and ensure that all wires are properly connected to the right connection points and none of them make contact between themselves. If the electrical system of your tow vehicle doesn’t require repairs, you should concentrate all your efforts on your trailer.

Use a MultiMeter to Troubleshoot Trailer Lights

To test your trailer’s plug, you will need a jump box or battery and alligator clip test leads. Keep in mind that if your trailer has electric brakes, you’ll also need a floor jack in order to jack up one wheel at a time. Whether you are using a jump box or battery, you will have to connect one end of the black test lead to the negative terminal. The other end of that test lead should be connected to the “Ground” pin.

Then you will have to connect the second test lead to the positive terminal of the jump box or battery and test all functions on the trailer by touching each of the pins with the opposite end of the second test lead. If none of the functions work, the “Ground” pin is probably the cause of the problem. Some wiring issues can be hard to detect, but knowing where to start and how to test trailer lights is half the battle.

About Andrew Demaree

Andrew Demaree is Bully’s resident technical writer and automotive expert. Being SAE certified mechanic, Andrew knows how your vehicle works and how to make it run even smoother. He is also passionate about truck customization and off-roading, so when it comes to adding some style and functionality to your truck, you can rely on Andrew’s knowledge.